On the Relationship Between Certain Indicators of the Structural-Mechanical and Water-Physical Properties of Dispersed Materials

ON THE RELATIONSHIP BETWEEN CERTAIN INDICATORS OF THE STRUCTURAL-MECHANICAL AND WATER-PHYSICAL PROPERTIES OF DISPERSE MATERIALS

V. M. Vitogin, V. A. Lotov

(Presented by the scientific seminar of the Department of OChT)

From the standpoint of physicochemical mechanics, the processes of plastic forming and granulation by the rolling method are deformation processes. The magnitude of the deforming forces is determined to a considerable extent by the water-physical and structural-mechanical characteristics of moistened disperse materials.

In processes of plastic forming, the structural-mechanical properties of the semi-finished product can be assessed fairly completely by the value of plastic strength ($P_m$) [1]. With the aid of curves showing the dependence of the plastic strength of a structure on its absolute moisture content, it is also possible to determine such important characteristics of the semi-finished product as the optimum forming moisture content and its formability. At the optimum moisture content, the structure of the semi-finished product acquires the most clearly expressed mechanical properties of an extremely concentrated suspension.

In the technology of granulating disperse materials, the suitability of the raw material is evaluated by its lump-forming ability [2]. The capacity of materials for wet aggregation is determined to a considerable extent by the nature of these materials, which in turn is characterized by such complex parameters of the water-physical properties of disperse materials as the maximum molecular ($W_{\mathrm{MMB}}$) and capillary ($W_{\mathrm{MKB}}$) moisture capacity. The first of these characterizes the complex of surface properties and the energetic state of the system; the second, the structural state of the material being aggregated. It should be noted that the value MMB in the present work is used—

not in the sense that A. F. Lebedev assigns to it, but as the moisture content characterizing the physicochemical state of the mass at the lower limit of plasticity—a somewhat blurred boundary, beyond which the principal role in retaining moisture passes from sorption forces to capillary forces; it is denoted below by the symbol \(W_{\mathrm{NKV}}\), the lowest capillary moisture capacity. The totality of the natural properties of disperse materials is assessed quite fully by the aggregability index \(K\), which may be determined from the following relation:

\[ K=\frac{W_{\mathrm{NKV}}}{W_{\mathrm{MKV}}-W_{\mathrm{NKV}}}. \tag{1} \]

Analysis of the regularities of wet aggregation of disperse materials showed that the basic condition for the successful implementation of the granulation process must be strict correspondence between the moisture capacity and the optimum moisture content of the material being aggregated, \(W_{\mathrm{opt}}\) [3]. It may be represented in the form

\[ W_{\mathrm{opt}}=\frac{W_{\mathrm{NKV}}}{K} \tag{2} \]

or

\[ W_{\mathrm{opt}}=W_{\mathrm{MKV}}-W_{\mathrm{NKV}}. \tag{3} \]

All the foregoing indicates the existence of two methods for determining the optimum moisture content for the same processes of interaction of disperse materials with water, which makes it possible to establish the interrelation between the water-physical and structural-mechanical characteristics of disperse materials.

For experimental verification of this proposition, we investigated batches composed of finely ground Tatar bauxite (fraction 80 microns), white electrocorundum sludge (fraction 50–0 microns), and technical alumina.

The plastic strength of the structure, formability, and the optimum molding moisture were determined by the method of S. P. Nichiporenko [1]. The maximum molecular moisture capacity \((W_{\mathrm{NKV}})\) was determined by the method of moisture-capacity media [4], and the capillary moisture capacity \((W_{\mathrm{MKV}})\) by capillary absorption of distilled water by columns of dry disperse materials 15 mm in diameter and 100 mm high.

The results of the investigations are presented in Table 1, from which it is evident that, for batches composed of bauxite and EB sludges, the values of the optimum molding and aggregation moistures are in good agreement. For bauxite–alumina and bauxite–alumina–sludge batches, some divergence of the results is observed owing to the specificity

structure of the clay-soil particles [5].

It is known that the plastic strength of a mass is inversely proportional to its absolute moisture content [6]. Consequently, for the inflection point of the curve \(P_m = f(W)\), corresponding to the optimum value of the molding moisture content and plastic strength, one may write the expression

\[ P_{m_{\mathrm{opt}}}=C\frac{1}{W_{\mathrm{opt}}}, \tag{4} \]

where
\(W_{\mathrm{opt}}\)—optimum molding moisture content, kg/kg;
\(P_{m_{\mathrm{opt}}}\)—plastic strength of the mass corresponding to it, N/cm\(^2\);
\(C\)—coefficient of proportionality characterizing the cohesiveness of the mass, N/cm\(^2\);

Processing of the experimental data made it possible to establish, between the coefficients \(K\) and \(C\), a dependence of the form

\[ C=\varepsilon\frac{K}{1-K}, \tag{5} \]

where \(\varepsilon\)—coefficient of proportionality, N/cm\(^2\).

The value of the coefficient fluctuates within the limits \(0.98 \div 1.02\) and, for practical calculations, may be taken equal to unity.

Substituting expressions (2, 5) into (4), we obtain

\[ P_{m_{\mathrm{opt}}}=\varepsilon\frac{K^2}{(1-K)\cdot W_{\mathrm{nkv}}}. \tag{6} \]

Expression (5) may be represented in the form

\[ C=\varepsilon\frac{W_{\mathrm{nkv}}}{W_{\mathrm{opt}}-W_{\mathrm{nkv}}}. \tag{7} \]

Consequently, for any moisture-content values somewhat greater than \(W_{\mathrm{mmv}}\), one may write

\[ C_i=\varepsilon\frac{W_{\mathrm{nkv}}}{W_i-W_{\mathrm{nkv}}} \tag{8} \]

or

\[ P_{m_i}=\varepsilon\frac{W_{\mathrm{nkv}}}{W_i-W_{\mathrm{nkv}}W_i}. \tag{9} \]

Expressions (6, 9) make it possible to establish the interrelationship between the water-physical properties of a dispersed system and the plastic strength of its structure, which, evidently, can reflect a complex of structural-mechanical characteristics of the molded semi-finished product. In addition, using expression (9), one can, by calculation, establish the dependence of the plastic strength of a mass on its absolute moisture content within the limits of the yield boundaries, and also determine its moldability at \(W_{\mathrm{opt}}\).

Table 1

Composition and basic structural-mechanical and water-physical characteristics of the investigated charges

The calculated and experimental data presented for comparison in Table I show a satisfactory correlation between the structural-mechanical and water-physical characteristics of moistened disperse materials.

References

1. S. P. Nichiporenko. Physicochemical Mechanics of Disperse Structures in the Technology of Building Ceramics. “Naukova Dumka,” Kiev, 1968.

2. V. M. Vitogin, A. S. Bogma. Izv. Vuzov, “Chernaya Metallurgiya,” 1969, No. 4, p. 18.

3. V. M. Vitogin, A. S. Bogma, P. N. Dokuchaev. Izv. Vuzov, “Chernaya Metallurgiya,” 1969, No. 8, p. 42.

4. A. M. Vasil’ev. Fundamentals of Modern Methods and Techniques for Laboratory Determinations of the Physical Properties of Soils. Moscow, Gosizdat for Construction and Architecture, 1953.

5. V. L. Balkevich. Technical Ceramics. Gosstroiizdat, 1968.

6. R. I. Zlochevskaya. Bound Water in Clay Soils. Moscow State University Press, Moscow, 1969.